Electrical contact devices, particularly for high switching frequency and high current loading



p 15, 1957 o. ROMER 2,789,181

ELECTRICAL CONTACT DEVICES, PARTICULARLY FOR HIGH SWITCHING FREQUENCY AND HIGH CURRENT LOADING Filed Nov. 19. 1954 United States Patent ELECTRICAL CONTACT DEVICES, PARTICU- LARLY FOR HIGH SWITCHING FREQUENCY AND HIGH CURRENT LOADING Otto Riimer, Berlin-Siemensstadt, Germany, assignor to Siemens & Halske Aktiengesellschaft, Berlin-Siemensstadt and Munich, Germany, a German corporation Application November 19, 1954, Serial No. 470,060

Claims priority, application Germany December 3, 1953 6 Claims. (Cl. 200-166 My invention relates to electrical contact devices and, in a more particular aspect, to devices for performing frequently recurring contact making and breaking operation under high current loads. Examples of such devices are the synchronous switches in mechanical contact converters and rectifiers used in power circuits that operate periodically at a switching frequency of 50 or 60 cycles per second.

The mutually engageable contact pieces in these devices are supposed to show extremely little wear even after a very large number of switching operations, such wear being caused by migration and dissipation of the contact material. It is further required that the contact resistance and the resultant voltage drop in the contact device always be kept at a low value to prevent undue heating and ensuing destruction of the contact pieces.

The material preferably used as contact material in such devices is silver, because this metal has a very low contact resistance. However, when operating at extremely high switching frequencies, the use of silver results in excessive migration and evaporation of material as well as in excessive frictional wear. The frictional wear caused by the mechanical impacts of the contact closing operations increases the contact resistance, and this results in heating the contacts to a high temperature so that they become damaged or destroyed after unsatisfactory short periods of use. The known silver alloys for control purposes also have excessive electrical wear when used at high switching frequencies and are generally inferior ,because of their high specific electrical resistance.

Contact materials of silver compositions with tungsten, molybdenum, nickel and other base materi-als have also been used. These materials have the disadvantage that due to the heating of the contacts, the non-noble constituents become oxidized so that the contact resistance increases excessively. Furthermore, the non-noble constituents such as tungsten, molybdenum, nickel, cadmium, zinc, tin, lead, cadmium oxide, zinc oxide, lead oxide and others are not corrosion resistant, so that their presence in the contact material results in increased contact resistance.

It is an object of my invention to obviate the abovedescribed deficiencies of the known contact materials occurring especially in contact devices subject to high switching frequencies in conjunction with heavy current loads. 7

To this end and in accordance with one feature of my invention 1 provide a contact piece consisting of a carrier or terminal body of good conducting material such as copper, silver and/oigold, and a surface layer joined with the carrier body and forming the active contact face of the piece; and I form the surface layer of a sintered composition comprising about 20% to about 70% gold, and 80% to 30% of high-melting noble metal selected from iridium and rhodium.

An example of a switching device equipped with contact pieces according to the invention is schematically Patented Apr. 16, 1957 illustrated in the accompanying drawing representing a partly sectional view. The illustrated device has two stationary contact pieces each having a relatively large carrier body 1, 2 of copper, silver or gold and a surface layer 10, 20 intimately joined with the carrier body by soldering, brazing or welding. The contact pieces are mounted on an insulating support 3. The two stationary pieces are periodically interconnected by a movable contact piece also composed of a carrier body 4, for instance of silver, and a surface layer 40 intimately joined with the body 4 in the afore-described manner. The movable contact piece is urged by a spring 5 toward bridging engagement with the stationary contact pieces and is actuated in opposition to the spring 5 by an axially displaceable tappet 6 controlled by a cam 7 on a continuously revolving shaft 8. For operation as a contact rectifier the shaft 8 revolves at the frequency of the alternating current to be rectified. The electric contact is made and broken between the surface layer 40 and the surface layers 10 and 20 and, as mentioned, these surface layers are composed of a sintered alloy or agglomeration of the highconductance metal, namely gold, with the high-melting noble metal namely indium and/ or rhodium.

The above-mentioned constituents, namely the gold and the noble metal, form together the bulk or substantially all of the composition, that is these two constituents together amount preferably to more than 70% of the entire composition. As will be mentioned below, the composition may also comprise a balance of other base metals and/or small amounts of paladin-m or platinum.

The corrosion resistant high melting noble metals iridium and rhodium have a relatively small specific electric resistance. These two high-melting noble metals, moreover, are highly resistant to electrically caused attacks as well as to mechanical wear. In combination with gold, the relatively high electrical conductance of or burning away of the materials are still further improved by the good electrical and thermal conductance of the.

gold constituent as this conductance secures good heat dissipation and correspondingly low operating temperatures. For that reason, the contact devices according to the invention are particularly useful in high frequency switching operations at higher Current and power loads than similar devices comprising known contact materials. Compared with the known contact materials, the compositions used according to the invention result in a much longer period of useful life of the contact devices and permit operation of the devices at higher switching frequencies.

Examples of materials that I found particularly suitable for the surface layer of contact pieces according to the invention are given in the following table, the numerals therein signifying percent by weight:

Table 1 Au Ir Rh Os cal methods.

' position.

3 or more of tungsten, rhenium, molybdenum, tantalum, nickel, chromium, cadmium, aluminum, berilliutn, man ganese, silicon and others. Further examples of such compositions, containing nickel as the added base metal are presented in Table II.

As mentioned, the compositions for the surface layers according to the invention are of the sintered type. That is, the compositions are produced by powder metallurgl- One way of production is to mix the components in pulverulent form, to compress and mold the mixture and then to fuse and solidify it by subjecting the molded body to sintering temperature. Another way of. producing the composition is to first produce a sintered porous body or skeleton structure of the high-melting noble metal, then. placing the proper quantity of gold onto the molded body, and thereafter heating the body in vacuum to a temperature above the melting point of the high-conductance metal or metals so that thelatter metals penetrate into and form an alloy with the sintered skeleton structure of the high-melting noble metals.

If the surface layer of the contact piece is to have a particularly high density or hardness, one or more metals that form hard alloys with gold may be added to the com- Such additive'metals are, for instance, pal ladium and platinum.. The amount of such additions is preferably about 1% up to 20%. r 7

When the contact pieces according to the invention are to be used under conditions where the contact surface is subjected to extremely high mechanical stresses, the mechanical or frictional wear can be reduced by heating to the contact composition a slight amount of metal selected from indium, tin, cadmium or zinc in accordance with. the invention disclosed in my copending application, Serial No. 470,059, filed November 19, 1954, for Electric Contact Devices, assigned to the assignee of the present invention. Preferably used for this purpose is.

' indium, but tin, lead, cadmium or Zinc can be used instead or in combination with indium. Preferred limits for additional indium are about 1% to 9%, for cadmium about 3% to for tin or lead about 10% and for zinc about The total amount of such additions should not exceed about 20%. The added quantity of iridium, tin lead, cadmium or zinc is placed upon the contact face of. the surface layer and the contact piece is then. ternpered in order to cause the metal to diffuse into the surface layer. In this manner, the surface layer is converted into a superficial zone in which the above-described composition of high-conductance metal and noble metal forms an alloy with the indiumor other metal used. However, the above-mentioned metals of the indium group may also be mixed with the constituents of the composition before sintering the composition, or they can be electroplated onto the contact face of the surface layer, preferably with a subsequent heating treatment for the production of a diffusion zone.

I claim: a

1. A contact piece of low electric resistance, particularly for frequent switching operation: under heavy current loading, comprising a carrier 'body' of good conductr ing material and a surface layer joined with said body to provide a contact face for switching operation, said surface layer being formed. of. a sintered compositioncomprising about 20% to about of gold, and 80% to 30% of metal selected from the group consisting of iridium and rhodium.

2. A contact piece of low electric resistance, particularly for frequent switching operation under heavy current loading, comprising a carrier body of good conducting material and a surface layer joined with said body to provide a contact face for switching operation, said surface layer being formed ofa sintered composition having a skeleton structure of about 30% to about 80% of metal selected from the group consisting of iridium and rhodium, and 70% to 20% of gold interspersed in" and alloyed with said structure.

3. A contact pieceof low electric resistance, particularly for frequent switching operation under heavy current loading, comprising a carrier body of good conducting material and a surface layer. joined with. said. body to provide a contact face for switching operation, said larly for frequent switching operation under heavy cur-- rent loading, comprising a carrier body of 7 good conduct-.

ing material and a surface layer joined with said body 'to provide a contact face for switching operation, said surface layer being formed of a sintered composition comprising about 20% to about 70% gold, and 3.0% to 80% of constituent substance selected from the group consisting of iridium and rhodium, and a remainder con sisting essentially of about 1% to about 20% of metal selected from the' group consisting of indium, tin, lead, cadmium and zinc.

5. A contact piece of low electric resistance, particularly for frequent switching operation under heavy current loading, comprising a carrier body of good conduct: ing material and a surface layer joined with said body to provide a contact face for switching operation, said surface layer being formed of a sintered compositioncomprising about 20% to about 70% of gold, 30% to 80% of substance selected from the group consisting of provide a contact. face for switching operation, saidsurface layer being formed of. a sintered composition comprising 20% to 65% of gold, 5% to 30% of nickel, the. remainder consisting essentially of 30% to of metal elected from the group consisting of iridium and. rhodium.

References Cited in the file of this patent UNITED STATES PATENTS 1,990,277 Feussner et al. Feb. 5, 1935 2,048,648 Feussner et a1. July 21, 1936 2,241,262" Keitel May 6, 1941 2,417,967 Booe Mar. 25, I947 

